Electron microscope observations on the ovarian follicle of the domestic fowl during the rapid growth phase.

A survey has been made of the ultrastructural features of the oocyte surface and its enveloping layers, comprising the theca, the granulosa and the perivitelline layer, during the final growth phase of 'yellow yolk' deposition. The following observations suggest that many macromolecular components of the blood plasma have free access from the vasculature to the oolemma: the fenestrated structure of the endothelium of the thecal capillaries, the intercellular spaces containing several erythrocytes in the theca interna, the 20-30 nm particles in the granulosa basal lamina, the wide spaces between the granulosa cells, and the open meshwork of fibres in the perivitelline layer. Numerous coated pits and vesicles, of 0.25-0.35 micron diameter, in the highly convoluted surface layer of the oocyte provide a mechanism for the incorporation of yolk precursors by pinocytosis. Such large coated vesicles and wide spaces between the granulosa cells are lacking in follicles in the preceding growth phase, which is concerned with the deposition of 'white yolk'. Considerable metabolic activity of the granulosa cells is indicated by their prominent Golgi elements, diverse granules, vesicles and villus processes. Cell junctions at the tips of the macrovilli anchor the granulosa to the oocyte. The theca externa, which provides mechanical support for the oocyte and its vascular and nervous elements, consists of sheets of collagen fibrils and fibroblast-like cells. Microfilaments in these cells may contribute to the contractility of the theca. Groups of interstitial cells associated with nerve fibres are present in the theca interna.